The active metabolite of acetaminophen is conjugated to glutathione (GSH) in the liver and excreted in the bile. Administration of H3-GSH before a toxic dose of acetaminophen results in the biliary excretion of radioactivity with a single chromatographic peak. These studies may lead to a better understanding of acetaminophen metabolism in animals and better methods of preventing acetaminophen-induced hepatic necrosis in man. More importantly, they may lead to a useful technique for evaluating the turnover, metabolism and synthesis of endogenous GSH in normal and diseased patients, a capability that is impossible at the present time.